2 ．シリア国における長期都市下水灌漑が農業環境に及ぼす影響
シリア国のアレッポ都市近郊地域における都市下水の長期灌漑利用の環境影響評価を 行った．特に，都市下水の混入した灌漑水の水質および土壌の重金属汚染程度を明らかに した．2009～2010 年に毎月都市下水が大半を占めるクウェイク川から採水を行い，生活 環境項目｛pH，浮遊物質量，全窒素，全リン，生物化学的酸素要求量（BOD5），化学的 酸素要求量（COD），大腸菌群｝を分析した．重金属｛カドミウム（Cd），クロム
（Cr），銅（Cu），ニッケル（Ni），鉛（Pb），亜鉛（Zn）｝の汚染程度は 25 年以上都 市下水を利用している農地並びに同程度の灌漑歴を持つ地下水灌漑農地の表層土（0～10 cm）の全重金属含量および可給態含量並びに逐次抽出法および選択溶解法による化学的 形態を測定した．クウェイク川は年間を通じて，シリア国基準と比較し，高い浮遊物質量，
全窒素，BOD5 および COD 濃度並びに多くの大腸菌群を含んでいた．この地域における 慣行的な小麦栽培の灌漑（5,000 m3 ha–1）により，シリアの小麦施肥基準の窒素は全量お よびリンは半量賄うことが可能であると見積もられた．この地域は都市下水の灌漑利用に より高い養分供給能並びに人体への生物的汚染の危険性が高いと考えられた．都市下水灌 漑農地においては全 Cr，Pb およびZn 含量並びに可給態 Cd，Cu，Ni およびZn 含量が地 下水灌漑農地よりも高かった．重金属の化学形態の分析結果より，都市下水灌漑農地の Fe 酸化物結合態画分の重金属量は地下水灌漑農地よりも高く，Ni を除き全ての重金属の 有機物結合態画分も高かった．そのため，これらの画分が都市下水灌漑農地における重金 属の長期間の吸着源として働いている可能性が考えられた．都市下水灌漑農地においては
全 Cr，Pb，Zn 含量の増加とともに Fe 酸化物結合態および有機物結合態の重金属が増加
していた．これらの増加率は Fe 酸化物結合態が高く，重金属の汚染の進行に伴い，Fe お よび Mn 酸化物が重金属の吸着源として有機物より安定的に働いたと示唆された．これは 測定した重金属の酸化物への吸着特性および結晶化度の低い Fe 酸化物の比表面積が大き いことによる重金属の吸着能が高いことに起因すると考えられた．さらに，Mn 酸化物へ の吸着性が高い Pb は易還元性の画分に多く含まれていた．長期間還元状態が維持された 場合，結晶化度の低い Fe 酸化物や Mn 酸化物が溶解するため，これらに吸着された重金 属が土壌中に溶出される危険性が示唆された．したがって，長期都市下水灌漑農地におい ては不適切な表層灌漑などの土壌の長期間還元状態維持によって，集積した重金属を溶出 させないことが必要であると考えた．重金属は有害物質であると同時に一部は微量必須元 素であり，長期間の都市下水灌漑による乾燥地土壌中の重金属動態の把握は今後の都市下
水の持続的かつ安全な農業利用を確立する一助になると考えられた．
以上のことをまとめると，都市下水は有用な水資源であり，養分供給量も莫大である ことが明らかになった．その一方で，長期の連用により窒素やリン，重金属が蓄積してい くため，都市下水灌漑農地においては都市下水によって供給される量に基づいた管理が必 要であることが明らかとなった．
Summary
This work aimed to evaluate and support establishment of safe wastewater usage for agricultural production. The studies specifically focused on clarifying the quantities of wastewater for potential and actual utility as an irrigation water resource, and the environmental impact in the most challenged region for the wastewater irrigation, the Middle East and North African region.
1. Regional (Middle East and North African region) and global estimation of wastewater generation and treatment
Despite several studies and reviews highlighting the significance of wastewater irrigation, information on the quantity of wastewater being produced and treated at global and regional scales is limited. Our objective was to estimate global wastewater generation and treatment volumes by using the available data in published or electronic forms. The estimations were calculated by multiple regressions. The factor for generated wastewater used by urban population, economy and urban precipitation indices. The indicators for treated wastewater were calculated by urban population, economy, sanitation and urban development indices. Model efficiency for wastewater production was 0.92, and 0.75 for wastewater treatment. The data are sufficient to estimate for countries for which no data available. Total global wastewater generated was estimated at 460 km3 in the year of 2000 and 579 km3 in the year of 2010. The estimated wastewater generation was equivalent to 20% of global water withdrawal for agriculture. The global nitrogen and phosphorus in the wastewater were estimated at 26.8 Tg yr–1 and 4.8 Tg yr–1, respectively. Thus, wastewater is recognized as having high potential for agricultural use as both a water and mineral nutrient resource in water scarcity areas and developing countries.
In the water scarce Middle East and North Africa region, the wastewater production was estimated at 18.6 km3 (8% of the regional water withdrawal for agriculture) in 2000 and 23.5 km3 (12%) in 2010. The untreated or inadequately treated wastewater was the typical disposal form in the region. Farmers in the region have relied on the untreated or insufficiently wastewater as irrigation water source. Therefore, irrigation with untreated or insufficiently wastewater has a concern for public health, especially heavy metal and biological contamination.
2. Effect of long-term wastewater application on the environment in Aleppo peri-urban areas, Syria
Though, several studies on wastewater irrigation have been conducted in the Middle East and North African region, information on irrigation with untreated or inadequately wastewater is completely lacking in some Middle East and North African countries, including Syria. This study
carried out an environmental impact assessment of long-term wastewater irrigation in Aleppo peri-urban areas. This study focused on water quality and the risks of nutrient loading and biological contamination, and heavy metal contamination in soils.
Water sampling from the Qweik River was done on monthly basis between 2009 and 2010.
The analytical parameters were pH, total suspended solid (TSS), total nitrogen (TN), total phosphorus (TP), biological oxygen demand (BOD5) , chemical oxygen demand (COD) and coliforms. The surface soils (0-10 cm) were taken from the area that had been under wastewater irrigation for more than 25 years in peri-urban Aleppo, Syria, for assessing heavy metal ((cadmium (Cd); chromium (Cr); copper (Cu); nickel (Ni); lead (Pb); and zinc (Zn)) distribution in soils. The soils were analyzed for the soil heavy metals using total and available content, and chemical forms by sequential extraction methods.
Results showed high content of TN, TP, TSS, BOD5, COD, and coliform in the Qweik River throughout the year. Under the current irrigation regime for wheat in the area (5,000 m3 ha–1) irrigation water is estimated to supply all the nitrogen and over half the amount for phosphorus requirements that would usually be applied in form of commercial fertilizer. These findings suggested that the wastewater irrigation at the study site would pose high public health risks and a high-potential for nutrient-load. Total Cr, Pb and Zn in the wastewater irrigated soils were significantly higher than those of groundwater irrigated soils. The soil heavy metal fractionation data showed that the residual fractions of all the heavy metals were more or less the same in wastewater and groundwater irrigated soils. However, most of the heavy metals in wastewater-irrigated soils significantly increased in bound forms to oxides of Mn and Fe (MnO/FeO) or with soil organic matter fractions (OM) as compared to groundwater-irrigated soils, except for Ni (p<0.05). The ratio of increase for MnO/FeO to total content was higher than those of OM. This was likely caused by poorly crystalline Fe oxide with relatively higher surface area which is capable of adsorbing the heavy metal. The Pb with higher potential to be absorbed by Mn oxide accumulated in the easily reducible fraction.
Summarily, this work clarified that global wastewater generation is equivalent to about 20% of global water withdrawal for agriculture and to 20% of nitrogen and 10% of phosphorus demand in the world. The long-term of wastewater irrigation in peri-urban areas of Aleppo showed that that there is high potential for nutrient load being converted to the fertilizer, however, with a high risk for biological contamination, likely due to low efficiency of the wastewater treatment plant in Aleppo. In addition, long-term wastewater led to accumulation of heavy metals in the soil, mainly adsorbed onto Mn and Fe oxides. The MnO/FeO fractions are relatively less bioavailable and less mobile, but would pose risks of the heavy metal release from the solid phase into liquid phase, under anaerobic conditions.
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